There are valid arguments on both sides of the stranger/friend debate. Ultimately it will come down to what you're both comfortable with (and what that is may change over time). My two cents is that swinging with an "unknown" guy is no more dangerous than dating the same guy in the vanilla world.
For our part we've had no luck with anonymous single guys (they sometimes get cold feet too) and a tough time recruiting casual acquaintances - they love the idea of banging my wife 'til she tells them she's swinging not cheating and that I'll know what goes on between them. Somehow that scares them off, it doesn't make any sense to me
. We started in our teens with my best friend, long before we knew anything about
"The Lifestyle". It was a great way FOR US to start because, as you mentioned, I trusted him which made everything much easier from a jealousy standpoint.
As for the peeks and valleys of her interest in a MFM, I can only suggest that you keep the fantasy going by talking about it, especially in the bedroom!
Good luck!